This invention relates to high voltage integrated circuits and more particularly to integrated circuit polysilicon field plates that are formed simultaneously with polysilicon resistors, both of which are insulated from high voltage metal crossovers.
Linear high voltage circuits are used in the telecommunications industry. In central office telephone exchanges the voltage is typically 70 volts. Such circuits include RC frequency-discriminating filters.
Integrated circuits for such applications must employ high voltage transistors and be constructed to avoid silicon surface inversions under high voltage conductors. Guard rings or the more space conserving field plates prevent such unwanted inversion.
Polysilicon resistors capable of high voltage operation are typically formed on an integrated circuit over a protective silicon dioxide layer. A description of such resistors and a method for making them is found in the patent to Miles et al, U.S. Pat. No. 4,256,515, issued March 17, 1981 and assigned to the same assignee as is the present invention. Such resistors are desirably made of thin high resistivity polysilicon strips.
Heavily-doped, low-resistivity polysilicon field plates are known for preventing inversion of the semiconducting regions between low voltage field effect transistors in a common pocket, e.g. in a memory array. Such field plates are made highly conductive by heavily doping them with impurities.
High voltage integrated circuits have employed highly conductive metal field plates surrounding a PN junction at the surface of the chip to expand the depletion region there and increase the breakdown voltage at that surface. Such field plates are typically connected electrically to the underlying doped region that is surrounded.
It is an object of this invention to provide a high density high voltage integrated circuit.
It is a further object of this invention to provide such an integrated circuit having polysilicon resistors and polysilicon field plates having identical thicknesses and identical dopant profiles that may be made by the same process steps at the same time.
It is another object of this invention to provide such an integrated circuit having a double insulating layer of silicon dioxide over the polysilicon field plates and resistors that in turn may be crossed by high voltage metal runs.
It is yet another object of this invention to provide such an integrated circuit that includes space conserving MNS capacitors for use in on-board filters wherein the nitride dielectric may be formed simultaneously with the nitride in the dual insultion covering the polysilicon.